SAN FRANCISCO (Reuters) - Software startup Slack Technologies Inc is raising $250 million in a new funding round, according to people familiar with the matter, boosting the company's valuation as Silicon Valley companies enjoy a surge in venture capital investments.

Slack, which makes messaging and collaboration software for businesses, is raising the fresh funding at a valuation of slightly more than $5 billion, the people said, a step up from its previous $3.8 billion valuation.

The investment is being led by SoftBank Group Corp <9984.T> and venture capital firm Accel, a previous investor, said the people, who declined to be identified. SoftBank has been heavily investing in startups, and it recently launched a $93 billion Vision Fund for technology investments.

Slack's sizeable funding round reflects the trend of a growing number of $100 million-plus checks pouring into technology startups. In the second quarter this year, there were 34 venture capital deals of $100 million or more, nearly triple the 12 such transactions in the first quarter, according to data firm PitchBook Inc.

These large rounds have helped drive an uptick in startup funding since the end of last year, with venture capitalists investing $15.7 billion in companies during the second quarter this year, a 27 percent increase from the first quarter and making for the strongest quarter in a year, according to Thomson Reuters data.